Influence of π-electron conjugation outside the aromatic ring on the methyl internal rotation of 4-methyl-5-vinylthiazole

The microwave spectrum of 4-methyl-5-vinylthiazole was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Only the anti-conformer was observed. Due to internal rotation of the methyl group, doublets containing an A and an E torsional species were found for all rotational transitions. Hyperfine structures arising from the 14N nuclear quadrupole coupling were resolved. The complex spectral patterns were analysed and fitted using the XIAM and BELGI-Cs-hyperfine codes, yielding a barrier to methyl internal rotation of 107.0901(7) cm−1 and the quadrupole coupling constants χaa = −3.545(13), χbb− χcc = −1.563(24) and |χab| = 1.76(11) MHz. The experimental results were supported by data from quantum chemistry. Some calculations using the MP2 method yielded non-planar structures, where the vinyl group is tilted out of the thiazole ring plane. The inertial defect and the χcc constant were compared with those obtained for other aromatic molecules to support the planarity of 4-methyl-5-vinylthiazole. A comparison of the methyl torsional barrier with other thiazole derivatives suggests that electrostatic effects caused by the π-conjugated system of thiazole extended by the vinyl group are the reason of the low barrier of the methyl group.本工作采用工作频率范围为2.0~26.5 GHz的脉冲分子束傅里叶变换微波光谱仪（pulsed molecular jet Fourier transform microwave spectrometer），测定了4-甲基-5-乙烯基噻唑（4-methyl-5-vinylthiazole）的微波光谱。仅观测到反式构象（anti-conformer）。受甲基内旋转影响，所有转动跃迁均呈现包含A、E两类扭转物种的二重峰结构。由¹⁴N核四极耦合（14N nuclear quadrupole coupling）产生的超精细结构得以分辨。采用XIAM与BELGI-Cs-hyperfine代码对复杂的光谱图谱进行解析与拟合，得到甲基内旋转势垒为107.0901(7) cm⁻¹，以及四极耦合常数χₐₐ = −3.545(13)、χ_bb−χ_cc = −1.563(24)和|χ_ab| = 1.76(11) MHz。实验结果得到了量子化学计算数据的佐证。采用MP2方法（MP2 method）进行的部分计算结果显示，乙烯基偏离噻唑环平面，体系呈现非平面结构。将惯性亏损与χ_cc常数和其他芳香族分子的对应值进行对比，佐证了4-甲基-5-乙烯基噻唑的平面结构特性。将甲基扭转势垒与其他噻唑衍生物进行对比后发现，乙烯基拓展的噻唑π共轭体系所产生的静电效应，是甲基内旋转势垒较低的原因。